Boron nitride (BN) has become a very important industrial material in recent years. It has many desirable physical properties. It can be a wide band gap semiconductor with high thermal conductivity and chemical inertness or a thermal insulator. Boron nitride exits in at least two crystal structures, i.e. hexagonal and cubic. Boron nitride that has the cubic crystalline structure, commonly known as cubic boron nitride (or cBN) is of particular interest to the electronics industry. This is due to its ability to be doped as a semiconductor of p or n type and also for its stability at high temperatures.
One method of producing BN by chemically transporting boron by H.sub.2 +N.sub.2 plasma is disclosed in J. Mat. Sci. Let., 4 (1985) 51-54. Another method of producing a BN film by depositing boron on a substrate from a boron vapor source and simultaneously irradiating ion species containing nitrogen onto the substrate was disclosed by Japanese patent Kokai Publication No. 181,262/1985. Still another method of producing BN by evaporating boron with a hollow cathode discharge source while ionizing nitrogen with an electrode and irradiating ionized nitrogen onto a substrate on which radio frequency wave is applied to produce boron nitride on a substrate is disclosed by Inagawa, et al., Proceedings of Ninth Symposium on Ion Source Assisted Technology '85, Tokyo, 299-302 (1985). However, none of these conventional methods is capable of producing a cubic boron nitride film that is single phase and single crystal. The BN films produced by these methods frequently contain hexagonal boron nitride and/or amorphous boron nitride.
More recently, a method that claims growing cBN on silicon wafers by using a laser ablation technique is disclosed in U.S. Pat. No. 5,096,740 to Doll, et al. In the Doll method, it is claimed that single crystal cubic boron nitride films were epitaxially grown on a silicon substrate oriented along the (100) axis such that the resulting cubic boron nitride films are in epitaxial registry with the underlying silicon substrate.
Another recent work disclosed by Nakagama, et al. in U.S. Pat. No. 5,096,740 also claimed a method of producing cubic boron nitride films by laser deposition. The Nakagama, et al. method comprises irradiating an excimer laser on a target comprising boron atoms and optionally nitrogen atoms and depositing cubic boron nitride on a substrate which is placed facing the target wherein a discharge of a gas comprising nitrogen atoms is formed between the target and the substrate.
Neither the Doll, et al. method nor the Nakagama, et al. method produces cBN films of high purity and high crystallinity. Furthermore, the bond formed between the silicon substrate and the cBN film is weak such that failure of the bond occurs in applications, especially at high temperatures.
It is therefore an object of the present invention to provide a method of forming cubic boron nitride films on silicon substrates that does not have any of the drawbacks of the prior art methods.
It is another object of the present invention to provide a method of forming single crystal cubic boron nitride films on silicon substrates that utilizes a pretreatment step on the substrate by hydrogen atoms.
It is yet another object of the present invention to provide a method of forming single crystal cubic boron nitride films on silicon substrates by utilizing a reactive biased laser ablation technique.
It is a further object of the present invention to provide a a method of forming single crystal cubic boron nitride films on silicon substrates such that films of high purity and high crystallinity can be produced.
It is another further object of the present invention to provide an improved method of forming single crystal cubic boron nitride films on silicon substrates utilizing a reactive laser ablation technique in which a biased ring is used to further energize a laser plume.
It is yet another further object of the present invention to provide a semiconductor device comprising a silicon substrate coated with a single crystal cubic boron nitride film by an improved reactive biased laser ablation technique.